The present invention relates to a class D amplifier suitable as a power amplifier for an audio system.
The class D amplifier is an amplifier that generates a pulse train with its pulse width or time density modulated according to an input signal and drives a load by way of the pulse train. The class D amplifier is often used as a power amplifier for driving a speaker in an audio system. In this type of power amplifier, clip occurs in an output signal waveform in case the level of an input signal exceeds a proper range. Such clip must be prevented because, once given to speakers as a load, it is output from the speakers as an unpleasant sound. In the related art technology, a level adjuster has been arranged before a power amplifier in order to optimize the amplitude of an input signal to the power amplifier thus preventing occurrence of clip. This technology is disclosed as “related art technology” for example in JP-A-2003-332867.
In the related art technology, the amplitude of the input signal to the power amplifier is compressed according to its level thus preventing clip. The problem is that the signal input to the power amplifier is accompanied by nonlinear distortion caused by the nonlinear characteristic of the level adjuster and the nonlinear distortion appearing on the output signal waveform degrades the quality of the sound regenerated from the speakers.
According to the technology disclosed in JP-B-3,130,919, variable gain amplification means is provided in the preceding stage of the PWM (Pulse Width Modulation) modulator of a class D amplifier. In case an input signal that is likely to generate clip is given to a PWM modulator, the source voltage of a switching amplification stage arranged after the PWM modulator is switched to a high voltage and the gain of the variable gain amplification means before the PWM modulator is decreased to prevent clip. The problem with the technology disclosed in Patent Reference 1 is that means for controlling the switching amplification stage is required to prevent clip thus resulting in a larger circuit. The technology disclosed in Patent Reference 1 switches the source voltage of the switching amplification stage to a high voltage when clip is likely to occur. Thus, the peak level of the output signal of a class D amplifier may become unnecessarily high. This does not fully satisfy the needs for audio reproduction at an appropriate sound volume and with reduced distortion.